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Article Summary

Higher plant populations can be managed by planting in narrower row spacings.

As plant population increases, the size of each individual root system becomes significantly smaller, which increases the need for better crop management especially fertility.

When growing corn at higher plant populations and/or narrower row spacings, it is important to select a hybrid that has a positive yield-response to these more intensive management practices.

Corn yields have increased significantly since the 1930s largely due to genetic improvement and better crop management. Grain yield is the product of the number of plants per acre, kernels per plant, and weight per kernel. Of the three components that make up grain yield, the number of plants per acre is the factor that the grower has the most direct control over. Kernel number and kernel weight can be managed indirectly through proper fertility, weed, pest and disease management to optimize plant health, and weather also plays a major role. Currently the average U.S. corn planting population is just under 32,000 plants per acre and has increased 400 plants per acre per year since the 1960s. If this trend continues, the average U.S. corn planting population will reach 38,000 plants per acre in 15 years and 44,000 plants per acre in 30 years.

Narrower Row Spacings

Today, the vast majority of corn in the U.S. is planted in 30” row spacings, with narrower rows generally defined as any row spacing or configuration less than 30” row spacings.

The most common narrower row spacings include 20” and 15” rows, along with twin rows that are spaced 7.5” apart (22.5” between rows, but are on 30” centers). Narrower row spacings can be used to increase plant-to-plant spacing within a row to reduce crowding at higher plant populations, thereby, allowing the crop to better utilize available light, water, and nutrients
(Figure 1).

In 2017 and 2018, six commercial DeKalb hybrids were planted at 38,000, 44,000, 50,000, 56,000 plants per acre in a 30” and 20” row spacing at Yorkville and Champaign, IL.

Figure 1. At the same plant population of 44,000 plants per acre, greater plant-to-plant spacing is achieved in the 20” row spacing compared to the 30” row spacing.

The management system that resulted in the highest grain yield of 294 bushels per acre was planting 44,000 plants per acre in a 20” row spacing (Table 1). The minimum plant population that maximized grain yield in a 30” row spacing was 38,000 plants per acre. On average, across plant population, plants in a 20” row spacing yielded 12 bushels per acre more than when planted in a 30” row spacing, however, as plant population increased the yield advantage of the 20” rows over the 30” row spacing was greater. Planting 56,000 plants per acre at either row spacing was too high of a population and yield decreased without a sufficient amount of resources such as water or nutrients to support that many plants. Evidence suggests that there is a limit on how high planting population can be pushed in either a 30” or 20” row spacing without any additional fertilizer, crop protection, or irrigation.

Better Crop Management

Management systems that decreased plant-to-plant spacing within a row, such as wider row spacing and higher plant population, decreased the size of the root system. On average, for every additional 6,000 plants planted per acre there was a 15-18% decrease in the size of the root system (Figure 2). However, when planted in a 20” row spacing, the greater plant-to-plant spacing increased the size of the root system by 22%. At higher plant populations, not only are there more plants that require nutrients and water, but each of those plants also have a significantly smaller root system. Crop fertility becomes even more important under these more intensive growing conditions. Placing nutrients directly in the root zone at the right time using the correct source and rate increases the probability that roots will take up and utilize those nutrients.

For every additional 6,000 plants planted per acre there was a 15-18% decrease in the size of the root system.

Figure 2. Individual plant root size decreases as plant population increases. At a given plant population, the 20” row spacing has a larger root system compared to the 30” root system.

Select the Right Hybrid

Hybrids vary greatly in their response to plant population and to narrower row spacings (Table 2). Hybrids also vary in their plant architecture and leaf trait characteristics. Understanding which hybrids better tolerate higher plant populations and narrower row spacings along with the plant growth and leaf traits that these hybrids possess would help lead the breeding effort for selecting hybrids that will perform even better in these management systems. Hybrids that produced greater yields in response to narrower row spacings and higher plant populations tended to possess the following plant growth and leaf traits: 1) greater above-ground biomass, 2) high leaf area index, 3) upright leaves, 4) thin leaves, and 5) less leafy plants.

Table 1. Grain yield as influenced by plant population and row spacing for corn averaged across six corn hybrids grown at Yorkville and Champaign, IL in 2017 and 2018.

Table 2. Grain yield and profit difference between planting 38,000 plants per acre in a 30” row spacing compared to 44,000 plants per acre in a 20” row spacing for six DeKalb corn hybrids grown at Yorkville and Champaign, IL in 2017 and 2018. Profit was calculated using $3.50 corn and $320.00 per bag of corn seed.

Summary

As the trend of increasing planting populations continues, it is important to consider the effects that the reduced plant-to-plant spacing has on the corn plants. Crop management becomes even more important, especially fertility, under these crowded conditions. Narrower row spacings can be used as a tool to reduce the plant-to-plant competition at higher planting populations.